The present invention relates generally to a system for detecting a displacement between an objective lens and a surface to be measured and more particularly, to the system using the optical astigmatic method.
The non-contact measuring method such as the optical astigmatic method, critical angle detecting method, or knife edge method is conventionally known, which serves to measure a displacement between the objective lens and the surface to be measured, or a shape and roughness of the surface. A displacement detecting system using such measuring method includes, for example, a non-contact surface roughness tester which uses an absolute level of a focus error signal to detect a displacement between the objective lens and the surface to be measured, and a focusing-type non-contact displacement detecting system which includes a drive arranged to the objective lens so as to make the latter follow the surface through feedback control of a focus error signal, detecting a displacement between the two through the drive.
FIG. 9 shows a constitution of the displacement detecting system disclosed in JP-A 4-366711. This system is a focusing-type non-contact detecting system with a scale. Referring to FIG. 9, an optical detector, designated generally by reference numeral 1, is arranged to detect through light reflected by a surface 6 to be measured a deviation of a distance between an objective lens 5 and the surface 6 from a focal length. An actuator 16 is arranged to move the objective lens 5 in the direction of an optical axis 21 in response to an output of a light receiving element 7 of the optical detector 1 so that the distance between the objective lens 5 and the surface 6 is equal to the focal length. A linear scale 20 is disposed on an axis 22 which is in line with the optical axis 21 of the objective lens 5, and integrally mounted to the actuator 16 so as to move together with the objective lens 5. The optical detector 1 comprises a laser diode 2, a beam splitter 3, and a mirror 4 in addition to the light receiving element 7.
The conventional displacement detecting system operates as follows: The optical detector 1 detects a deviation of the distance between the objective lens 5 and the surface 6 from the focal length through light reflected by the surface 6, and the actuator 16 moves the objective lens 5 in the direction of the optical axis 21 so that the distance between the objective lens 5 and the surface 6 is equal to the focal length in response to an output of the light receiving element 7 of the optical detector 1. The linear scale 20, disposed on the axis 22 which is in line with the optical axis 21 of the objective lens 5 and integrally mounted to the actuator 16, is moved together with the objective lens 5 in accordance with a displacement of the surface 6. By this, a displacement of the surface 6 can be detected as a displacement of the linear scale 20.
With the conventional displacement detecting system, however, the optical detector 1 needs at least the laser diode 2, beam splitter 3, mirror 4, and light receiving element 7, and is constructed by the combination of the above parts, resulting in a complicated constitution and increased size of the system. Further, due to the need of fine adjustment, the system has a weak point for a dislocation and vibration, resulting in unsuitable mass production.
Further, when the drive is arranged to the objective lens 5 so as to be variable with respect to the surface 6, a collimator lens is needed in addition to the objective lens 5, which may produce a dislocation upon mounting, resulting in a potential occurrence of an error factor.
Furthermore, due to lengthened optical path of the optical detector 1 upon position detection, the component parts of the optical detector 1 undergoes an influence of thermal expansion, etc. by temperature variation, resulting in a difficult measurement of the displacement amount of the surface 6 with accuracy higher than on the order of 1/100 [.mu.m].
It is, therefore, an object of the present invention to provide a system for detecting a displacement between an objective lens and a surface to be measured with high accuracy and a reduced size.